Being able to search for particular keywords or phrases in commit summaries or diffs can greatly help in tracking down a change.
It may be feasible to stuff custom entries into the searchindex table (w/ MySQL search) or indirectly into the search engine (w/ Lucene), but this may require infrastructure for a clean search engine update interface.
Being able to restrict the search by branch or path, date, or committer would be suitably useful with this. Identifying classes/methods/functions altered even when their definition lines don't appear in the diff would be *very* helpful, but might be harder. (Break these out to separate bugs if not initially built with the other stuff.)